Definition: In this task, you're given an article and an answer. Your task is to generate the fill-in-the-blank question for this answer based on the given article. The blanks are always at the end of the question, and can be filled with more than one word or phrase.
Input: Article: Believe it or not, if you do not use your arms or your legs for a long time, they become weak. When you start using them again, they slowly become strong again. Everybody knows that. Yet many people do not seem to know that memory works in the same way.  When someone says that he has a good memory, he really means that he keeps his memory in practice by using it. When someone else says that his memory is poor, he really means that he does not give it enough chance to become strong.  If a friend says that his arms and legs are weak, we know that it is his fault. But if he tells us that he has a poor memory, many of us think that his parents are to blame, and few of us know that it is just his own fault.  Have you ever found that some people can't read or write but they have better memories? This is because they cannot read or write and they have to remember things; they cannot write them down in a little notebook. They have to remember days, names, songs and stories; so their memory is the whole time being exercised. 
 Answer: his memory is not often used

The main reason for one's poor memory is that   _  .